TECHNICAL PAPER #12
MEAT AND EGG
Dr. H.R. Bird
Leonard Z. Eggleton
1600 Wilson Boulevard, Suite 500
Arlington, Virginia 22209 USA
703/276-1800 . Fax: 703/243-1865
Understanding Poultry Meat and Egg Production
Volunteers in Technical Assistance
This paper is one of a series published by Volunteers in
Assistance to provide an introduction to specific
technologies of interest to people in developing countries.
The papers are intended to be used as guidelines to help
people choose technologies that are suitable to their
They are not intended to provide construction or
details. People are
urged to contact VITA or a similar organization
for further information and technical assistance if they
find that a particular technology seems to meet their needs.
The papers in the series were written, reviewed, and
almost entirely by VITA Volunteer technical experts on a
Some 500 volunteers were involved in the production
of the first 100 titles issued, contributing approximately
5,000 hours of their time.
VITA staff included Leslie Gottschalk
and Maria Giannuzzi as editors, Julie Berman handling
and layout, and Margaret Crouch as project manager.
Dr. H.R. Bird, the author of this paper, is a professor
and former chairman of the Department of Poultry Science at
University of Wisconsin.
He has taught poultry nutrition, feeding,
management, and general animal nutrition at the Universities
of Wisconsin and Maryland.
He has also consulted on these topics
in Brazil, Indonesia, Belize, and Nepal. Leonard Z. Eggleton
the Chairman of Agricultural Projects with the Iowa-Yucatan
Peninsula Partners project at Iowa State University.
consulted on poultry in Uruguay and Mexico.
Ralph Ernst is a
Poultry Specialist with the cooperative extension program of
Department of Avian Sciences, University of California at
He has worked with game bird, duck, and turkey
Pinkston is a returned Peace Corps Volunteer who worked in
husbandry in the Philippines, which included developing a
program for poultry, engaging in incubation of eggs, and
VITA is a private, nonprofit organization that supports
working on technical problems in developing countries.
information and assistance aimed at helping individuals and
groups to select and implement technologies appropriate to
maintains an international Inquiry Service, a
specialized documentation center, and a computerized roster
volunteer technical consultants; manages long-term field
and publishes a variety of technical manuals and papers.
UNDERSTANDING POULTRY MEAT AND EGG PRODUCTION
VITA Volunteer Dr. H.R. Bird
Since ancient times, chickens, ducks, and geese have served
farming communities by gleaning the fields of grain that
would be lost; picking up grain that is dropped by the
wayside in threshing, drying, and transportation; making
use of the scraps from the family table; and, supplementing
those feed items by foraging for grass, weed seeds, and
With such a diet these animals are able to produce eggs and
which provide protein of high quality plus several essential
vitamins and mineral elements.
Eggs and meat are ideal supplements
for the cereal grains, tubers, and roots that provide much
of the energy in many human diets.
Besides being recoverers of waste grain and users of scraps
by-products, poultry can function to provide a food
farming community that can do so would like to produce more
than the people need.
Maybe the excess can be sold; but, if not,
it can be fed to poultry.
Then if there is decreased production
of grain in a certain year, the poultry flock can be
instead of decreasing the grain that is supplied to the
Small flocks of poultry--from a few birds to a few
the rule all around the world, until the 20th Century.
early 1900s, flocks numbering in the thousands began to
North America and Europe.
In the 1920s and 1930s, geneticists,
nutritionists, physiologists, and disease specialists
improved breeds and strains of chicken and improved methods
feeding and managing them and protecting them against
The rapid introduction of new technologies so increased the
of producing eggs and poultry meat that costs to consumers
went down at a time when prices for most other consumer
This paper addresses the following important questions to
you decide whether poultry raising is for you:
How can poultry flock owners in developing
of modern technology?
Is it better to use native birds or import
Is it possible with local feedstuffs to
and the efficiency of feeds based on corn
Can vaccines, coccidiostats, antibiotics,
supplements be imported economically?
of them be
Is it possible to make feeding and watering
PRODUCTION: VARIATIONS AND ALTERNATIVES
FREE-RANGING POULTRY VERSUS CONFINED POULTRY
When one thinks of free-ranging poultry that find their own
require no care, and provide food for the family table, one
However, there are disadvantages too.
communities that take a serious interest in their poultry,
some degree of confinement.
Letting poultry range freely is an economical way to provide
with feed. On the
other hand, it exposes them to predators.
Moreover, they cannot be guaranteed a balanced diet from
foraging. To achieve
a well-balanced diet, they must be periodically
supplemented with hand-fed food.
Free-ranging poultry are not crowded and therefore may be
susceptible to disease, but Newcastle disease--a virus that
plagues poultry--can exterminate even a free-ranging
and the protozoa that cause coccidiosis live everywhere that
Regardless of whether they are confined or free-ranging,
chickens must be vaccinated or medicated against these
diseases as well as many others.
It is much easier to vaccinate
and treat confined flocks.
Free-ranging poultry do require less
labor than confined poultry, but in finding waste feed and
of water they are more likely also to find parasites,
Free-ranging poultry incubate their own eggs and thus
themselves, but they may lay eggs in unexpected places so
some are lost.
Furthermore, the process of becoming broody and
incubating eggs decreases the rate of egg production.
improved laying strains are non-broody and often will not
incubate their own eggs.
Confining poultry and providing sanitary feeders and
have a number of advantages:
better control of diseases;
protection from predators;
more efficient collection of eggs; and
easier access to poultry.
The disadvantage of confinement is that poultry can neither
nor forage. In the
Orient, to overcome this problem, poultry
attendants drive their flocks of ducks to the rice fields
then return them to their living quarters.
Similarly, a flock of
chickens can be confined in a house and yard next to a
floor, for example, and let out when there is grain to
Particularly when birds are confined, poultry production can
There are three distinct production systems
to consider, and each has gone through dramatic changes.
These systems are designed to produce chicken eggs, chicken
and duck meat. There
have not been parallel developments in
Most geese are still kept in small flocks and
depend on grazing and gleaning for much of their feed.
The changes in the system involve changes in the birds
in their feed, in disease control (vaccines, medicines,
sanitary practices) and in equipment and management.
What kinds of birds are best for the enterprise?
This is the
first question the small producer (200 birds or less) will
to answer. In many
parts of the world there are varieties of
local birds that have been selected, to some degree, for
production of eggs and meat.
There are also available, in most
areas, chicks of egg strains and meat strains that have been
developed by selection and strain crossing in North America,
Europe, Japan, and Australia.
Imported strain crosses are always
more productive and more uniform than improved local
they are also more expensive and cannot reproduce
Flock owners must continue to buy chicks, for as long as
this kind of stock.
Unfortunately, there appears to be no published information
the levels of productivity available with local breeds of
given the advantages of modern feeds, sanitation and
Such birds are kept in small confined flocks for egg
production, and both floor pens and battery cages are used
this purpose. It is
important to cull such flocks to eliminate
the poor egg producers.
The comb and wattles of a good layer are
large, soft, warm, and red.
The vent is enlarged and moist and
the pubic bones are spread apart.
They can be felt, to the
right and left of the vent.
A poor layer or non-layer will have
shrunken, pale, dry comb and wattles, a small dry vent and
spaced pubic bones.
Commercial production of eggs and broilers, with flocks
in the thousands, is now widespread and depends entirely on
strain crosses rather than local varieties.
Reports from India
and Pakistan emphasize the importance of imported strain
in the development of their commercial production in those
The establishment of a small flock of ducks is not likely to
involve the same choices as in the case of chickens.
situations one would have to depend on locally available
a local strain.
Poultry feeds usually consist of combinations of energy and
protein sources, which make up 90 percent or more of the
feed. The remainder
of the feed consists of calcium and phosphate
supplements and salt, which make up two to eight percent;
trace mineral, vitamin, and amino acid supplements, which
one or two percent, or sometimes more.
In the United States, for
example, a feed would consist of corn (maize), which is an
energy source and supplies some protein; soybean meal, which
is a very good protein source and supplies some energy;
(for calcium); dicalcium phosphate (for phosphorus and
salt; methionine (an amino acid not abundantly provided by
meal); and trace mineral and vitamin supplements.
mineral and vitamin supplements are shipped all over the
and are priced so low that they usually are not major cost
Cereal grains and legumes, unlike mineral and vitamin
are costly to produce and often in short supply in many
Due to their scarcity and to the competition
with human food supplies, their use for poultry feed in the
Third World is usually kept to a minimum.
We noted above that poultry served early farming communities
utilizing scraps and otherwise wasted food materials.
poultry can also utilize by-products of food
processing. There is
a prevalent notion that modern high-producing strains of
must have modern high-protein, high-energy diets.
chicken still functions well on by-product diets even though
is descended from many generations of ancestors that were
high energy, high protein corn-soy diets.
To illustrate, strain-cross
layers in an experiment at the Univeristy of Wisconsin
maintained 67 percent of egg production on the following
meal 1.0 percent
limestone 5.4 percent
salt 0.5 percent
hydroxy analogue 0.1 percent
trace - mineral 1.0 percent
limestone grit ---
The vitamin-mineral supplement provided, per kilogram (kg)
International Units (I.U.) of vitamin A, 900 International
Chick Units (I.C.U.) of vitamin D3, 22 I.U. of vitamin E,
10 milligrams (mg) of riboflavin, 0.7 mg of folic acid, and
mg of zinc carbonate.
Costa (1981) observed good performance with a broiler
feed of the following composition:
and polishings 32.5
meal, solvent process 17.0
bone meal 15.0
Vitamin-trace mineral supplement
The vitamin trace mineral supplement provided, per kilogram
diet: 8000 I.U. of
vitamin A, 1000 I.C.U. of vitamin D3, 5 I.U.
of vitamin E, 6 mg of menadione sodium bisulfite, 4 mg of
30 mg of niacin, 12 mg of d-pantothenic acid, 301 mg of
choline chloride, 20 micrograms of vitamin B12, 100 mg of
mg of zinc (as zinc oxide), 50 mg of manganese (as manganous
oxide), 0.25 mg of iodine (as ethylene diamine
mg of iron (as iron sulfate), and .10 mg of selenium (as
The two formulas given above are examples of poultry rations
would be economically feasible in some areas.
It is beyond the
scope of this report to provide formulas for a wide range of
or to present a treatise on feed formulation.
gives the requirements of different classes of poultry for
energy, protein, calcium, and phosphorus; and Tables 2 and
respectively, give the levels of these nutrients in various
ingredients and mineral supplements.
Using this information, one
could calculate formulas to supply these four
almost always has to add 0.5 percent of salt (NaCl); since
ingredients do not supply it.
One must also use a vitamin-trace
mineral supplement similar to the one used for adult birds
young growing birds in the two diets presented earlier.
1. Nutrient Requirements of Broilers, Laying
Hens, Growing Ducks, and Growing Geese
(*) Kilocalorie: a
unit of heat energy equal to 1,000 calories.
2. Composition of Feed Ingredients (as fed)
Type of Feed
Meat and bone meal
Mustard seed meal
Palm nut meal
Sunflower seed meal,
Sweet potato meal
Table 3. Composition of Mineral Supplements
By-product feeds vary greatly depending on the method of
Processing methods are well standardized in developed
but may be highly variable in developing countries.
resulting products may also be highly variable and quite
from those listed in Table 2.
Some feed ingredients have special disadvantages that must
meal contains gossypol, which discolors egg
yolks and inhibits growth of young birds.
Cooking the meal during
processing decreases the free gossypol and results in a
that is usually satisfactory for growing birds but may still
Mustard seed meal contains a growth inhibitor
and should not represent more than five percent of the diet.
Rapeseed meal contains a goitrogenic compound that
with thyroid function, and also should not represent more
five percent of the diet, unless improved strains of the
Soybeans contain an inhibitor of one of the important amino
acids, trypsin, which, interferes with digestion but can be
by cooking. The
processing of soybean meal is now so well
standardized that this inhibitor is seldom a problem.
To know whether soybean meal is cooked thoroughly enough,
this simple procedure:
Place 10 teaspoons (about 30 grams) of the
meal in a
small jar with
a tight lid.
Add 1 teaspoon (about 4 grams) of fertilizer
urea and 5 teaspoons of water.
Stir the contents and cover the jar with the
Wait 20 minutes.
Sniff for the odor of ammonia.
If ammonia is present, the soybean meal
and has not been heated enough.
Field beans (navy, pinto, kidney, etc.), like soybeans,
growth-inhibiting material which can be destroyed by
The information provided here is intended to help in
the feasibility of starting a poultry production enterprise
important source of food and income.
Now you must ask yourself:
What feeds are available locally, at what volume, and at
price? Are they, or
substitutes, available year round? Can
be combined to make a suitable formula, or will other
have to be shipped in from other areas?
If you are considering a
medium-sized or large-scale operation, you should get local
The final test of the quality of the
ingredients and the formulation is how well the poultry
No poultry business can succeed very long unless measures
taken to control diseases.
With this in mind, here are some
general guidelines that will help in maintaining a healthy
Feeding poultry a well-balanced diet will
deficiency diseases. To illustrate how
is, note that a marked deficiency in the
retard growth, decrease the rate of egg
and lower resistance to infections.
Whether your poultry flock is large or
small, it is a
good idea to
keep it separated as much as possible from
poultry. Do not encourage
visitors. Do not
help" with neighbors who have poultry.
buy adult or
half-grown birds and add them to your
If a flock is purchased, birds should be
for a period
of 5 to 15 days for observation.
birds separately from mature stock.
When selling birds, empty the poultry house
Clean it thoroughly, wash with a disinfectant
lye), and let
it stand empty for four weeks before putting
in new birds.
Provide your poultry with clean, sanitary
For birds housed in floor pens, provide
litter such as
straw, sawdust, rice hulls, or similar
materials. Provide sufficient
ventilation to keep the
enough so the birds can scratch in it.
should not be
wet or sticky.
If your birds are housed in cages, the cages
with a slatted bottom to allow manure to
to the floor. For small numbers of
the manure can
be collected in pans, which must be
cleaned once or twice each week. The
composted and used to fertilize your crops or
fertilizer. The sale of recycled manure
source of income. For large flocks, the
arranged so that the manure falls on the
floor where it can be allowed to accumulate
months or possibly even one year.
accumulation are possible in dry rather than
climates. If manure becomes wet, fly
This is usually best controlled by weekly
(drying, composting, etc.) of manure.
It may also be
necessary to add an insecticide to the
the cages to prevent development of flies.
authorties should be consulted to learn which
Table 4 presents a general vaccination and medication
for chickens. It is
not necessary to follow the entire schedule
at all times in all locations.
In tropical areas, it is safe to
assume that Newcastle virus is present and to vaccinate
it. Furthermore, the
strains of the virus that occur in the
tropics are commonly more virulent and more damaging than
in temperate areas.
Therefore it is sometimes recommended that
poultry farmers use a Newcastle vaccine produced locally
than an imported product.
Fowl cholera and fowl pox are two common diseases found all
the world, but this does not necessarily mean that they are
prevalent in your areas.
So, inquire first before starting a
vaccination or inoculation program.
Fowl pox is caused by a
virus; fowl cholera is caused by a bacterium.
As shown in Table 4, vaccines are available against
bronchitis and Marek's disease.
Both of these are caused by
viruses, but are less likely to cause trouble than Newcastle
disease, fowl cholera, or fowl pox.
The microscopic protozoan organisms that cause coccidiosis
present wherever there are chickens. Young chickens
these organisms early in life and may show bloody diarrhea,
weight loss, sluggishness, and ruffled feathers. The number
deaths may be few or it may be many. Most survivors will
and carry some degree of resistance to the organism
Drugs known as coccidiostats protect against this disease
widely available. Maintenance of dry conditions in pens will
minimize this disease.
Chickens kept on the ground or in floor pens are always
to intestinal worms (ascarids). Chickens may carry
numbers of the parasites without showing disease symptoms,
heavy infestation decreases egg production.
Ducks are affected by fewer diseases than chickens. They may
harbor roundworms and tapeworms without showing symptoms.
infestations may cause problems if ducks have access to
water or muddy, poorly drained soil.
If disease is suspected, it is desirable to seek expert
including diagnosis and possible post-mortem examination.
EQUIPMENT AND MANAGEMENT
During its first week of life, a baby chick should have
a brooding area at a temperature of 32 to 35[degrees]C.
After the first
week, the temperature can be decreased 2 to 3[degrees] each
typical small square poultry house is about six or seven
on a side. It will house 400 broilers or 100 layers. Early
attempts to increase the size of poultry farms were achieved
increasing the number of houses, but it was obviously more
to increase the size of the house. However, even in
temperate climates with moderate rainfall it is difficult to
ventilate a house that is more than 13 meters wide. For the
humid tropics, 10 meters is probably the limit. The length
limited only by the topography of the land or the poultry
bank account. In the tropics, the house may well be open on
or both sides except for wire netting or woven wire. A house
open on both sides should be equipped with a canvas that can
pulled up or let down on the windward side in order to
drafts at night and during storms. The house should be
both ends, and it should have a floor and a gable roof,
should provide at least 0.8 meter of overhang on each side.
covered opening at the peak of the roof can be used to
Roosts are not required but are often preferred to make
easier. A laying house that has a solid wall on one side may
a row of roosts arranged against the wall. The front, or
row should be about 0.8 meters above the floor. Two or more
rows of roosts go between it and the wall, with each roost
slightly higher than the one in front. The area under the
may be closed with wire netting to prevent access by the
It then serves to collect most of the droppings without
giving the chickens access to them.
If both front and back of the house are open, movable roosts
be provided along the center line of the house, or to the
or back. If the house is more than about 20 meters long,
can be installed not only along the ends but also along
Nests should be about 30 centimeters (cm) square and 30 cm
They are usually arranged in rows two to three tiers high.
should be a perch below the entrance of each nest, and the
row of nests should be about 0.5 meter above the floor.
should be about one nest for every four layers.
Nests for ducks should be on the floor, one nest for each
five birds. Partitions between nests are 30 cm by 35 cm.
are fastened at 28-cm intervals to a 15-cm board at the back
along the house walls and have a 5-cm board along the bottom
front. This leaves the top and front open.
Feeding troughs can be made of bamboo, wooden boards,
metal. Mechanical feeders are available for large flocks.
can be made from bamboo or from recycled glass or metal
containers, or automatic watering devices may be purchased.
3 cm of feeder space per chicken in the first three weeks of
life, then 5 cm until they are eight weeks old, and 9 to 10
after that. A feeder 100 cm long provides 200 cm of feeder
In temperate and subtropical zones, it is customary to
artificial light for layers. A 14-hour day is optimal for
production. This may not be necessary in equatorial regions.
Java, for example, strains imported from the United States
achieve the same annual egg production without lights as
in the United States with lights. Day length varies by only
few minutes throughout the year in Java. However, at the
of Delhi, India, day length varies annually (from 10 hours
20 minutes to 13 hours 57 minutes) and artifical lighting is
In every poultry house, cleaning and refilling waterers and
feeders should be the first maintenance task in the morning.
Clean waterers every day, whether they are automatic or
If they are hand-filled, they must be filled often
enough so that water is always available. Feeders should
be empty, but they should not be overly full either. Adding
frequently encourages the birds to eat and prevents waste.
In a laying house, eggs should be collected at least four
day: morning, noon, afternoon and late afternoon. Making a
second collection in the morning would be even better.
The following additional daily chores are recommended:
Dispose of dead birds.
Observe nest boxes. Clean when necessary.
Remove wet litter around waterers.
Observe height of feed hoppers. Edge of
at level of birds' backs. Adjust when necessary.
Add limestone or oyster shell to hoppers
Sweep down wire netting.
Watch for evidence of rodents, and
Add disinfectant to foot bath or pad at
Watch for sick birds.
Observe condition of litter. Stir when
Observe light bulbs. Clean when necessary.
If electric fans are used for ventilation,
The following are recurring specialized jobs that require
Distribute day-old birds in the house.
Move pullets from growing house to laying house.
Catch broilers (or old hens) and send them
Vaccinate against poultry diseases.
Thirty years ago in the United States, two hours per year of
labor were required for each laying hen kept, and one hour
labor for each pullet raised. Now it is customary to
about seven minutes per year of labor for each laying hen
four or five minutes for each pullet raised. This dramatic
change resulted from mechanization, larger flocks, the
from floor pens to laying cages, and some miscellaneous
in the organization of the operation. In many parts of the
world, existing economic and social structures favor
rather than capital-intensive operations. In those
the labor requirement will lie somewhere between the
CARE OF EGGS AND MEAT
Gather eggs several times each day (see section on
Clean eggs with a clean, damp cloth or in an egg
washer. If an egg washer is used, the water should be
warmer than the temperature of the eggs and should contain a
detergent-sanitizer. Eggs should be as fresh as possible
consumed or sold. For whatever time they are held before
they should be placed small end down in a cool place,
Kill chickens the same day the meat is to be used unless a
refrigerator is available to keep the meat from spoiling. To
kill chickens, hang them by their feet and cut across the
in the throat with a sharp knife. Let all of the blood drain
into a container. The blood can be cooked, dried, and added
feed for other chickens.
To remove feathers, place the bird (after it has been bled)
water at 60[degrees]C. That temperature is well below
boiling, but too
hot to put your hand in. As soon as the feathers are well
with hot water, pluck them as quickly as possible.
De-feathering ducks is more difficult than de-feathering
Slightly higher scalding temperatures are used for ducks.
The temperature of the water should not be above
65[degrees]C and the
length of scald varies from one and a half to three minutes.
In hand-scalding, grasp the bill with one hand and the legs
the other hand and submerge the rest of the body, breast
in the water. The bird is then pulled repeatedly through
the water against the feathers.
Because costs vary so much from area to area, it is
make accurate generalizations. The operating cost breakdowns
egg production and broiler production in the United States
India are shown in Tables 5 and 6. In all cases, the major
item is feed. A large part of the increase in efficiency of
meat and egg production is the result of more efficient conversion
of feed to product. About 2 kg of feed are now required
to produce 1 kg of broiler; 50 years ago, 4.5 kg of feed
required. And while about 1.7 kg of feed are required now to
produce one dozen eggs, 50 years ago, 2.3 kg of fed were
Table 5. Egg Production
(Percent of Total)
(Percent of Total)
Table 6. Broiler Production Costs
(Percent of Total)
(Percent of Total)
The cost of baby chicks has gone up with inflation, but
improvement has increased productivity so that chick cost
unit of product has remained nearly the same.
It is difficult to compare labor costs for broilers. In the
United States, the contract grower furnishes some labor, and
is provided by special crews under
"miscellaneous." The feed is
delivered into an automatic feeding system, so some labor
included in the feed cost. High interest rates contribute to
high miscellaneous costs in India. In the United States,
costs may allow for depreciation under
"Contract grower," but the allowance appears to be
III. DESIGNING THE RIGHT SYSTEM FOR YOU
Table 7 summarizes the poultry requirements for small-,
and large-scale poultry operations. Note, however, that all
poultry farms, regardless of size, should try to use modern
disease control methods. Modern vaccines and medications are
widely distributed in many parts of the world.
Table 7. Requirements for Egg or Meat Production
According to Flock Size
200-1000 More than
200 Birds Birds
Local or imported
table scraps, local feed
formulated feed local
Homemade Homemade or
or Hired or
Even the smallest poultry farm can practice isolation and
Small operations may choose between local and imported
stock and between formulated feed and a feeding program
what is available from day to day. Large operations will
use imported stock and formulated feed.
Small units will use homemade buildings and equipment and
labor. Large units may choose either homemade or commercial
equipment and either hired labor or a combination of
and labor. In some tropical countries laying flocks
in the thousands are housed in homemade, two-level,
laying cages of bamboo and wood slats. Such cages do not
last long in the tropics, but they can be replaced at
Table 8 summarizes the requirements for different classes of
8. Requirements for Different Masses of Poultry
Local or imported Local or
Crop residues, Crop
residues, Crop residues
table scraps, table
scraps, recovered by
local by- local
by- herding in
products, or products,
or fields, table
formulated feed formulated
feed scraps, local
Homemade or Homemade
or Homemade or
Family, hired, Family,
hired, Family, hired,
mechanized or mechanized
Costa, M.A. "The Evaluation of Indigenous Feedstuffs
Swine and Poultry in Belize, Central America."
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Gupta, S. P., ed. Indian Poultry Industry Yearbook,
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Maurer, A.J., and Maurer, E.A. Raising Chickens in Eastern
Wisconsin-Nicaragua Partners and Centro para el
National Academy of Sciences. Atlas of Nutritional Data on
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National Academy of Sciences. Nutrient Requirement of
D.C.: National Academy Press, 1977.
North, M. O.
Commercial Chicken Production Manual. Second edition.
Connecticut: AVI Publishing Co., Inc., 1978.
Orr, H.L. Duck and
Goose Raising. Publication 532. Ontario,
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Piliang, W.G.; Bird, H.R.; Sunde, M.L.; and Pringle, D.J.
Bran as the
Major Energy Source for Laying Hens." Poultry